In Magnetic Resonance Imaging (MRI), Radio-Frequency (RF) coils are used as antennas for exciting the nuclear spins and detecting signals from them. With the availability of low cost multi channel RF amplifiers, the total number of transmit channels is not anymore limited by costs. High count distributed RF amplifiers can supply multi element transmit arrays with a large number of coil elements to satisfy the increasing requirements of high field clinical applications.
A difficulty is that each individual coil of a multi channel RF system produces its own RF field, which may induce dangerous levels of heating within a subject when a magnetic resonance image is acquired.
In international patent application WO 2006/117714 A2, a multi-channel transmit/receive antenna for use in magnetic resonance imaging is disclosed. RF amplifiers can be used to their full peak power capability without running the risk that the RF amplifier is damaged due to excessive reflected power at its output. The specific absorption rate is monitored by evaluating certain forward and reflected power signals. In WO 2006/117714 A2 the use of circulators for performing the measurement is disclosed. The circulators are however not able to be used under high magnetic field conditions, such as exist within the bore of a magnet used for magnetic resonance imaging.